Chemical equilibrium help? For which of the following reactions are both reactants and products likely to be found when the reaction appears to be complete? How can you tell? - Mg + O2 -- 2MgO - HF + H2O H3O(+) + F(-)

Question

Chemical equilibrium help? 
For which of the following reactions are both reactants and products likely to be found when the reaction appears to be complete? How can you tell?
- Mg + O2 --> 2MgO
- HF + H2O <--> H3O(+) + F(-)

anonymous · Answer

The second one. One way to tell is because if it is an equilibrium system is is usually written with the double arrow, like this: $$\color{green}\rightleftharpoons$$ Another possible way to tell is because the second equation involves an aqueous solution.